Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly vehicles. A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well.
An electric vehicle refers to a vehicle that has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter.
Regarding the hybrid vehicle or electric vehicle, it has been proposed to boost the DC voltage from the DC power supply with a voltage step-up converter and supply the boosted DC voltage to the inverter which drives the motor (for example, Japanese Patent Laying-Open No. 09-240560, Japanese Patent No. 2879486, Japanese Patent Laying-Open Nos. 2000-050401 and 08-240171).
Japanese Patent Laying-Open No. 09-240560 for example discloses a power feeding apparatus for an electric-power-assisted vehicle, including a human-power drive system supplying a pedaling force to drive wheels, an electric-power drive system supplying an auxiliary force from an electric motor to the drive wheels and auxiliary force control means for variably controlling the auxiliary force according to the pedaling force and the vehicle speed, and further including voltage step-up means for boosting a battery voltage to a voltage according to an auxiliary force that is requested by the auxiliary force control means and smoothing means for smoothing the boosted voltage and supplying the smoothed voltage to the electric motor.
The voltage step-up means is thus used to boost the battery voltage to a voltage that is necessary for obtaining a target auxiliary force. Accordingly, the battery weight as well as the battery cost can be reduced.
The voltage boosted by the voltage step-up means is further smoothed by the smoothing means. Thus, output current values from the battery are averaged and accordingly the battery life can be extended.
Conventional motor drive apparatuses generally employ a DC-DC switching power supply, as voltage step-up means for boosting a battery voltage to a voltage according to a necessary auxiliary force. The DC-DC switching power supply performs its switching operation in response to a voltage command signal that is input from a control circuit so as to boost a voltage to a desired voltage and output the boosted voltage. Here, the voltage command signal that is input to the switching power supply is generated, by a control circuit, by calculating an auxiliary force to be generated by the electric-power drive system based on detection signals from a pedaling force sensor and a vehicle speed sensor to determine a target electric-current value necessary for obtaining the auxiliary force and variably control a requested voltage so that electric current flowing through the electric motor is at a target electric-current value.
Thus, a sudden increase or decrease of a required auxiliary force has to be immediately addressed by the switching power supply. Then, the control circuit is required to have a high-speed performance that is enough to address the sudden change in load. For satisfying this requirement, higher accuracy and larger size of the control circuit are indispensable, resulting in a new problem in terms of device size and cost.